1. Field of the Invention
This invention relates to an image reader which reads image information recorded on a reflective original or a transparent original.
2. Description of the Related Art
A digital photoprinter photoelectrically converts image information recorded on a photographic film (e.g., a negative film, a reversal film or the like), printed matter or the like "converts", the read image information to a digital signal, processes the digital signal into image information for recording, and scans a photosensitive recording medium such as photographic paper by a recording light beam modulated according to the image information, thereby printing the image information.
In such a digital photoprinter, editing such as synthesis of a plurality of images or division of an image, editing of layout of printed image including characters and pictures, and various image processings such as adjustment of color and/or density, change of magnification, and emphasis of contour can be freely performed, and accordingly, a finished print freely edited and freely processed as desired can be output.
In the conventional surface exposure printing, all the image density information recorded on a photographic film or the like cannot be reproduced in terms of density resolution, space resolution and color/density reproducibility. However, with the digital photoprinter, a print in which image density information recorded on a film is reproduced substantially one hundred percent can be output.
Such a digital photoprinter generally includes an image reader which reads an image recorded on an original such as a photographic film, a setup system which processes the read image and determines a subsequent exposure condition, and an image forming system which scans a photosensitive medium with a light beam according to the exposure condition determined and develops the photosensitive medium.
In the image reader which reads an image recorded on a photographic film, for instance, a linear reading light beam extending in one direction is projected onto the film and the film is moved in a direction perpendicular to the one direction (or the reading light beam and a photoelectric element are moved in a direction perpendicular to the one direction) so that the reading light beam two-dimensionally scans the film.
Transmitted light which is transmitted through the film and carries thereon information on the image recorded on the film is focused on a light receiving surface of a photoelectric converter element such as a CCD (charge coupled device) line sensor, and the image information is photoelectrically read.
The read image data is amplified and converted to a digital signal, and-the digital signal is subjected to various image processings such as compensation for fluctuation in properties of the CCD elements, density conversion, magnification conversion and the like and then transferred to the setup system.
In the setup system, the transferred image information is reproduced as a visible image on a display such as a CRT.
The operator observes the reproduced image and makes required additional correction such as a gradation correction, a color/density correction and the like (setting of a setup condition). If the reproduced image is satisfactory as a finished print, the image data is transferred to the image forming system as an image information for recording.
In an image forming system wherein an image is recorded by raster scan, three light beams corresponding to photosensitive layers sensitive to the three primary colors, e.g., R, G, B, formed on the photosensitive medium are modulated according to the image information for recording and deflected in a main scanning direction (corresponding to the one direction) while the photosensitive medium is being fed in a sub scanning direction substantially perpendicular to the main scanning direction (the reflected light beams and the photosensitive medium are fed in the sub scanning direction relative to each other), whereby the photosensitive medium is two-dimensionally scanned by and exposed to the modulated light beams and the image on the film is recorded on the photosensitive medium.
The exposed photosensitive medium is then subjected to development appropriate for the kind of the photosensitive medium. For example, when the photosensitive medium is of silver-salt type, the exposed medium is subjected to color development, bleaching/fixing, water washing and drying and the output as a finished print.
In order to obtain a high quality finished print with such a digital photoprinter, a photoelectric converter element which is high in both space resolution and density (light intensity) resolution must be used, and, for instance, a CCD sensor is suitably used. The range of density D (log E) of an image which is recorded (or can be recorded) on a negative film is generally about 3.2 and that of an image recorded on a reversal film is about 3.8. Thus, an image having a wide density range can be recorded. In order to obtain image information with high accuracy and to output a high quality finished print, image information of high density resolution up to about 0.01 is necessary.
When image information on a negative film having a density range of 3.0 is read at a resolution of 0.01 using a photoelectric converter element which has a wide density range (dynamic range) and is excellent in both the space resolution and the density resolution, a high performance A/D (analog-to-digital) converter of 16 or more bits is required to quantize the obtained image information.
Such a high performance A/D converter has not been put into practice up to now and would be very expensive even if put into practice.
Accordingly, in conventional image reader employed in the digital photoprinter, the amount of information to be read is limited.
For example, by effecting pre-reading (pre-scan) for roughly reading an image on a film prior to image reading for printing with the dynamic range of the photoelectric converter element held wide and determining the range of density to be read by the CCD sensor, the amount of information to be read can be limited.
However, there is an increasing demand for a high quality finished print and it is necessary to faithfully process high accuracy image information recorded on a photographic film. At present, though it is possible to faithfully read image information on a photographic film, it is difficult to faithfully process the obtained image information at high speed in the stage of quantizing the information.
High accuracy image information can be quantized by using a plurality of A/D converters and effecting analog-to-digital conversion in a plurality of steps. Such an A/D converter system is shown in FIG. 4.
For example, when analog image information is to be converted to 16-bit digital image information, the analog image information is amplified to a desired level and high-order eight bits of the image information are first converted to digital image information by a first A/D converter 401. The digital image information is supplied to a D/A (digital-to-analog) converter 403 and reconverted to analog image information. The analog image information obtained by the reconversion is input into a subtractor 405. The subtractor 405 takes the difference between the original analog image information and the analog image information from the D/A converter 403 corresponding to the high-order eight bits. The difference outputs is input into a second A/D converter 407 and low-order nine bits are converted to digital image information. The digital image information of the high-order eight bits and the digital image information of the low-order nine bits are input into a digital operator 409 and are output from the digital operator 409 as 16-bit image information after subjected to an over-range calculation and the like.
With the 16-bit A/D converter system described above, a dynamic range of 96.3dB can be theoretically ensured and accordingly it is possible to faithfully read image information recorded on a photographic film. However since the A/D converter system is a series-parallel system, the conversion operation becomes complicated and requires a long time as the number of steps increases, whereby mis-code becomes more apt to occur.